Processes for the removal of low concentrations, especially concentrations below 2% by volume, of hydrogen sulphide from gas streams are known, in which aqueous alkaline scrubbing solutions containing metal ions or oxometallate species are used to oxidize the hydrogen sulphide and/or its dissociated ions to elemental sulphur during an absorption or scrubbing step, the solution being regenerable, through oxidation, for re-use. Examples of such processes are the Stretford and Takahax processes. Variants of and improvements in these processes are also known and some of these are described in U.S. Pat. No. 4,243,648, RSA Patent No. 83/0444, U.S. Pat. No. 4,206,194, UK Patent No. 2029386, RSA Patent No. 82/3577, U.S. Pat. No. 4,283,379 and RSA Patent No. 82/4576.
The metal ions or oxometallate species used in most of these processes are usually selected from a group consisting of iron, vanadium, copper, manganese and nickel, with vanadium being a preferred member of the group. The vanadium is usually present as complex vanadates and/or other oxometallate species, preferably in the five- or four-valent oxidation state. During the absorption step, the vanadium is reduced from the five-valent to the four-valent state, while the sulphur of the hydrogen sulphide is simultaneously oxidized to elemental sulphur. During the regeneration step, the four-valent vanadium is reoxidized to the five-valent state.
One of the problems encountered with processes employing vanadium is the relatively low solubility of four-valent vanadium in water or alkaline scrubbing liquors resulting in a tendency of the four-valent vanadium to precipitate during the absorption step. To overcome this problem, some conventional processes rely on complexing agents such as citric, maleic or tartaric acid. Another problem encountered is the relatively low rate of oxidation of the four-valent vanadium to five-valent vanadium in the regeneration step. Alternatively or additionally, an oxidizing agent or promoter, such as a salt of anthraquinonedisulphonic acid (ADA), is added in other conventional processes to assist reoxidation of the vanadium. Other promoters which may be used are organic nitrogen compounds, inorganic amines, and carbohydrates (see RSA Patent No. 83/0444). Non-quinone aromatic compounds and water soluble carboxylate complexing agents (U.S. Pat. No. 4,283,379) capable of maintaining tetravalent vanadium in solution have also been proposed. Some of these compounds could have a dual function, namely to promote the oxidation of the metallic species in the regeneration step, and to act as complexing agent to keep the tetravalent vanadium in solution in the scrubbing step.
RSA Patent No. 82/4576 describes a process for preventing losses of vanadium in oxidative hydrogen sulphide scrubbings with an aqueous alkaline scrubbing agent containing five-valent vanadium as the oxidizing agent, in which a minimum content of carbonate ions of 12.5 g/l is maintained in the scrubbing agent, with a preferred concentration of between 13 and 17 g/l. However, all the experiments conducted in support of this patent specification were carried out with a scrubbing agent which also contained ADA. It has been found that ADA, although acting as an oxidation promoter in the oxidation of the vanadium in the regeneration step, also acts as a complexing agent, thereby maintaining otherwise insoluble tetravalent vanadium in solution. This has been confirmed by experiments conducted by the applicant in the absence of ADA, in which it was found that, with at least some carbonate ion concentrations in accordance with the teachings of RSA patent No. 82/4576 but in the absence of a complexing agent, vanadium precipitated from the scrubbing liquor.
As will be appreciated by those skilled in the art, the aforementioned complexing agents and oxidation promotors are expensive, and could cause pollution problems associated with the effluent from plants in which such chemicals are used.
There accordingly exists a need for a hydrogen sulphide removal process and a scrubbing liquor regeneration process in which the aforementioned problems are at least partially overcome.